Introduction   Parts Used   Design   Other Costs & Details   Observations & Conclusions

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Jeep XJ/MJ Dual Transfer Cases

Introduction

Today, with the tough rocky trails, going slow is the name of the game. The problem is that there is usually a compromise involved:

• you gear your truck slow enough to stay happy on the trails and suffer the consequences on the pavement; or
• you keep good highway gears but are always wishing to be able to crawl slower off-road.

This was the dilemma I was facing. With 4.10 gears, the street 33" tires are ideal for the highway. But with 35", and now 36" off-road tires, my trail gearing left a lot to be desired. I wanted to avoid re-gearing the differentials; the cost is pretty steep and my highway gearing (which I use a lot) was perfect. The truck is my daily ride. In fact, it is my only ride! I was faced with four alternatives to re-gearing the differentials:

1. Install a different transmission/transfer case combination. I looked closely at this option. An NV4500 transmission would be nice, but they are hard to find and expensive to purchase. An NV4500 would also require an almost total rebuild of the floorboards due entirely to its size. Going to a 4-speed transmission with an ultra-slow first gear was also an option, but that would have meant losing overdrive, and I do too many highway miles. Plus, all of my options for cheap 4-speed transmissions would leave me with a front driveshaft that was too short for my lift. Having the transmission shifter come up into my dash wasn't particularly appealing, either.
   I could have chosen to switch to an automatic transmission, too. Larry Soo's automatic seems to work well for him off-road in the majority of conditions. But sourcing all the parts needed to switch my truck from a manual to an automatic transmission seemed like an expensive undertaking that would still leave me with rather high gearing (despite the torque converter).
2. Purchase a lower low-range gear-set for my NP231 transfer case. I didn't like this option because I was getting into non-standard "hard parts", where if I broke something, I would have to return to the original company. What if the company no longer exists when the need arises? What if they do still exist, but replacement parts are prohibitively expensive? Also, I'm not always going super-slow, and sometimes the lower low range might be too slow. This option was finally discarded for one simple reason: I considered the low-range gear-set too expensive for my wallet.
3. Buy the dual T-case "box" that is available. I liked this option because it gives me another range for gear selection and the option of 2wd low range. But again, it involved "hard parts" that could be difficult or expensive to source to replace. And its price is about the same as the lower low-range gear-set for the NP231.
4. Build my own dual transfer case "box". Sure, I would be getting involved in non-standard parts, but at least I knew what those parts were, so I could reproduce them and/or make them better when the need arose. Plus, it gave me a chance to learn about another aspect of my truck and likely (hopefully) save some money. This appealed to me, so this option was chosen.

   Editor's note: Rob forgot to mention that he is, by nature, a cheap bastard. It was obvious to me that he would take the do-it-yourself route.

This article describes the design, build-up, mistakes, and observations of home-brew dual T-case project. It is not a project for the timid, but it can certainly be duplicated.